Particleboard can be produced from dry, fine wood particles that are mixed with binders and formed into a mat, which is then pressed together under high temperature and pressure into a densified board.
Wood raw material of almost any type of species may be used. However, the properties of the finished board, such as, for example, density, glueability etc are dependent upon the properties of the wood.
Sawdust, shavings, chips and shavings from round wood, in this specification and claims, referred to as "particles", are used as wood raw material. Flaking of the round wood takes place in drum flakers, while chips are processed in knifering flakers.
Subsequent to disintegration all wood material is dried down to 2-4% moisture in high capacity dryers. After the drying process the wood particles are screened to the preferred size. Rejected material passes through hammer mills and is fed back to the screening system.
Both the form of the flakes/sawdust and their size distribution are of importance for the board properties.
The most commonly used binder for particleboard and medium density fibre boards (MDF), is urea-formaldehyde resin (UF), but also melamine-urea resin (MUF), phenol resin (PF) and isocyanate resin (MDI) are used to some extent, especially for production of weather resistant board.
Resin, water, hardener and wax emulsion are automatically dosed on weight base. Dosages of the chemicals are calculated on the dry substance in percent of dry wood material. The amounts of binder added varies depending upon the resin type and the quality of board desired.
The dosage of UF resin is normally between 7-10%, MUF-resin between 11-13%, PF resin between 6-8% and MDI resin between 2-5%. The comparatively low dosages of PF and MDI resins reflect the superior binding ability of these resins.
A normal particleboard consists of about 6% moisture, 9% binding agent and other chemicals and 85% wood. In spite of the fact that the totally dominating ingredient is wood, the research and development (R&D) efforts within the particle-board industry has, up to mid 80's almost exclusively been dealing with the binders and the role of the wood has been neglected.
It is well known in the pulp and paper industry that the wood must be stored for a certain time before the production of pulp takes place to avoid quality and process problems. During storage wood undergoes important changes in the chemical composition. For instance, some volatile compounds disappear, the amounts of free and bonded acids increase, unsaturated bonds oxidize, hydrolysis of esters will occur, etc.
The particleboard industry has, however, not paid these facts much attention, but instead concluded that process and quality problems are more likely to stem from variations in the binder quality.
Up to now it has not been possible to establish valid correlations between the analysis result of the wood material and the properties of the board, even though there would seem to be a certain connection between acid content and the processability of wood.
One object of the present invention is to accomplish an on-line, in-line or at-line measuring of the raw wood material flow into the plant, giving the possibility of sorting out unsuitable material before it enters the process line.
Investigations of raw wood particles with the analysis technique according to the present invention have now surprisingly shown very high correlations between measured analysis values of the wood and the board properties, e.g. with the board's content of free formaldehyde, which today is extremely important, considering the very stringent environmental stipulations as well as firmness, and water resistance.
Information on particleboard and the processes for the manufacturing thereof is available in "Modern Particleboard & dry-process fibreboard manufacturing" by Thomas M. Maloney (1993), (cf. especially Chapter 4 and 5), which by reference is incorporated herein in its entirety.
The principles of NIR spectroscopy are described by Williams, P.; Norris,K. (1987): New-Infrared Technology in the Agriculture and Food Industries. AACC, St. Paul/Minn. and Sterk, E.; Luchter, K. (1986): Near Infrared Analyses (NIRA) A Technology for Quantitative and Qualitative Analyses. (Applied Spectroscopy Revues 22:4.), all of which are hereby incorporated by reference.
The use of multivariate data analysis in the characterisation of multi-component systems is presently a field of development. Applied generally to the field of chemistry those statistic methods are also termed chemometrics methods. The technique of chemometrics is more fully explained in S. D. Brown, "Chemometrics", Anal. Chem. 62, 84R-101R (1990), which by reference is incorporated herein in its entirety.
The term "board" includes in this specification and claims the following board types: particleboard, medium density fibre board (MDF), waferboard, oriented strand board (OSB), hardboard and plywood.
Process variables which influence the quality of the board are e.g. the wood raw material, viz. sort of wood, the maturing level, the composition of the particles as well as size and moisture content; the particle generation such as Hombak/Mayer particles; the dryer, its inlet and cutlet temperatures, dried particle moisture; screening parameters such as surface and core particles, dust content, fractions, moisture content, particle temperature; glueblender variables such as surface and core particles, scale settings, particle temperature, glue amounts, wax amounts, moisture content, cooling water; forming station variables such as volume weight, thickness etc.; pre-press variables such as press time and temperatures; hot-press variables such as press time, pressure, temperature; cooling variables such as temperature; and sanding variables such as surface fineness.
In the proceedings from the 48th Appita Annual General Conference (held at Melbourne, Australia, 2-6 May 1994) Meder et al present an article entitled "Prediction of wood chip and pulp and paper properties via multivaritate analysis of spectral data" (pages 479-484). According to the conclusive part of the article (page 484) Meder et al have used PCR analysis of FTIR, NIR and NMR spectra of wood chip samples to predict the chemical composition of the chips (i.e. in fact to determine said composition from the spectra), and to attempt (although, as explicitly indicated in the article, not very successfully) to predict some physical properties of Kraft and TMP pulp and paper properties. The article does however not suggest any method for qualitative or quantitative determination of parameters of a wood based panel produced from raw wood material flowing into a process for production of wood based panels, little less any method for controlling any process variables in such a process.
In an article in the scientific magazine "Holz als Roh-und Werkstoff 50 (1992) p 25-28" Niemz et al. states that the quality of the board is influenced by the solid resin content and the relation hardwood/softwood. Niemz et al. use NIR spectroscopy for the quantifying of the portion of urea-formaldehyde resin at chips and the mixing ratio of hardwood to softwood. The aim of the tests as performed are to establish if the process is suitable to prove urea-formaldehyde on glued sawdust and to obtain the mixing ratio hardwood/softwood.
It is also stated in the said article that NIR-technique can be used in combination with a linear multiple regression for on-line and off-line control of wood moisture and for the analysis of chemicals and agriculture products. It is furthermore stated that Norris 1962 for the quantitative analysis of foodstuff and fodder combined NIR with mathematical-statistical methods (chemometrics) which later was used for the quantitative analysis within classical chemistry.
In another article in the same magazine at p 73-78 by Kniest a sawdust-glue mixture is characterized by NlR-spectroscopy in combination with linear multiple regression. However, it is at p 77 item 3, 2nd paragraph stated that the measuring of unglued samples is not possible due to the requested data allocations for the process modelling of each board. [Zur Durchfuhrung o.g. Industrieversuche ist die Messung der zugehorigen unbeleimten Probe aufgrund der furdie Prozessmodellierung notwendigen Datenzuordnung zur jeweiligen Spanplatte nicht moglich.]
It is obvious from the said reference that the man skilled in the art did not consider it possible to predict the properties of the board from the unglued particles nor to determine such properties in a non-destroying manner from the produced board, and the problem to find an efficient on-line, in-line or at-line method at the beginning of the process for the determination of the parameters characterising the board remained unsolved.
Relevant parameters defining the properties of board are e.g. density and density profile, internal bond, thickness swelling, absorption, permeability, perforator value, modulus of rapture (MOR), parameters relating to volatile organic compounds (VOC) and emission chamber values.
Density is in this connection the same as volume weight and is normally determined by weighing strips of the board with known volume and dividing the mass with the volume. Values are expressed in kg/m.sup.3.
Internal bond (IB) is the property of a given board to resist tension perpendicular to the plane of the board. The result is depending on the resin content and the board density; in both cases almost a linear function.
Thickness swelling is measured by placing a sample of a certain size in water with a temperature of 20 or 23.degree. C. during a period of 2-24h. Thickness of the sample is measured before and after the soaking. The thickness difference is divided by the original thickness and expressed in percent. Thickness swelling is a measure of the board's ability to withstand for example unexpected rain or water based paint etc.
Absorption value is normally performed with the same sample that was used for measuring thickness swelling. The sample is weighed before and after the water exposure. The weight difference is divided by the original weight and expressed in percent. The absorption value can be used to predict the board behaviour under severe conditions.
Permeability value is obtained by sucking air through the sample (the board edges are sealed with wax) and the pressure drop across the board is measured along with the air flow through the sample. The permeability varies over the board surface depending on variations in board density but normally there is a good correlation between average permeability and the formaldehyde emission value. Permeability measures the resistance the formaldehyde has in escaping from the board. Values are expressed in cm/minute.
Perforator value expresses the formaldehyde content of the board at a certain moisture content (6.5%). The formaldehyde is obtained by extracting the board in toluene. The released formaldehyde is absorbed in water and determined photometrically. As could be expected there is a connection between the perforator value and the formaldehyde emission from the board and the perforator method is therefore an approved method in many countries. Values are expressed in mg HCHO/100 g ovendry board.
The Emission chamber method is now accepted all over the world as the most accurate method for determination of formaldehyde release from wood based panels or other materials. The conditions in the chamber are set to simulate the conditions in a normal home. The size of the chamber varies between the countries from 1 m.sup.3 to 40 m.sup.3. The temperature varies from 23 to 25.degree. C., the load varies from 0.3 m.sup.2 /m.sup.3 to 1.0 m.sup.2 /m.sup.3, the relative humidity from 45 to 50% and the air rate exchange from 0.5 to 1.0/hour. The board samples are placed vertically with a certain distance in racks in the chamber. Air samples are taken until a steady state is reached, which normally takes 3 to 10 days. Values are expressed in ppm HCHO or in mg HCHO/m.sup.3.
Density profile is a measure of the mat forming function and the function of the press and also of the geometry and mix of the wood particles. The profile is today measured by use of apparatus with X-rays capable of measuring the density for each 0.1 mm from surface to surface. A normal density profile for particleboard shows surface densities of 1100 kg/m.sup.3 down to 600 kg/m.sup.3 in the core.
Thus, much research work has been done in the past to find a solution to the said problem during the years but no convenient solution has been available until by the present invention.